Drosophila melanogaster has one pair of sex chromosomes (XX or XY) and three pairs of autosomes, referred to as chromosomes II, III, and IV. A genetics student discovered a male fly with very short (sh) legs. Using this male, the student was able to establish a pure breeding stock of this mutant and found that it was recessive. She then incorporated the mutant into a stock containing the recessive gene black (b, body color located on chromosome II) and the recessive gene pink (p, eye color located on chromosome III). A female from the homozygous black, pink, short stock was then mated to a wild-type male. The F₁ males of this cross were all wild type and were then backcrossed to the homozygous b, p, sh females. The F₂ results appeared as shown in the following table. No other phenotypes were observed.

Based on these results, the student was able to assign short to a linkage group (a chromosome). Which one was it? Include your step-by-step reasoning.

Verified step by step guidance

1. Step 1: Understand the genetic background and the traits involved. The traits are black body color (b) on chromosome II, pink eye color (p) on chromosome III, and short legs (sh) of unknown chromosomal location. The mutant alleles are recessive, and the female used in the backcross is homozygous recessive for all three traits (b, p, sh). The male parent in the backcross is wild type for all traits.
2. Step 2: Analyze the F2 phenotypic classes and their counts. The phenotypes are Wild type, Pink only, Black and Short together, and Black, Pink, and Short together. Since the female parent is homozygous recessive for all three traits, the phenotypes in the progeny reflect the alleles contributed by the male parent and the recombination events between the genes.
3. Step 3: Determine linkage by comparing expected independent assortment ratios to observed data. If short legs (sh) were on a different chromosome from black (b) or pink (p), the phenotypes involving sh would assort independently, and the phenotypic ratios would reflect independent assortment. However, if sh is linked to either b or p, the phenotypes involving those traits will show non-independent assortment, with certain combinations appearing more frequently.
4. Step 4: Compare the phenotypic counts of 'Black, Short' and 'Black, Pink, Short' classes. If short legs (sh) is linked to black (b) on chromosome II, these two classes should be more frequent and appear together more often than expected by chance. Conversely, if sh is linked to pink (p) on chromosome III, the 'Pink' and 'Black, Pink, Short' classes would show linkage patterns.
5. Step 5: Conclude the linkage group of short legs (sh) by identifying which chromosome's recessive trait co-segregates with sh. The chromosome whose recessive trait appears together with sh more frequently than expected indicates linkage. Therefore, based on the phenotypic distribution, assign short legs (sh) to the chromosome that shows this pattern of linkage.

Key Concepts

Linkage and Chromosome Mapping

Linkage refers to genes located close together on the same chromosome that tend to be inherited together. By analyzing offspring phenotypes and their frequencies, geneticists can determine whether genes are linked and assign them to specific chromosomes. This concept is essential for interpreting the inheritance patterns in the F2 generation and identifying the chromosome carrying the short (sh) gene.

Recessive Traits and Pure Breeding

A recessive trait only appears phenotypically when an organism has two copies of the recessive allele (homozygous recessive). Pure breeding stocks are homozygous for the trait, ensuring consistent expression in offspring. Understanding this helps explain the initial crosses and the phenotypic ratios observed in the progeny.

Sex Chromosomes vs. Autosomes in Drosophila

Drosophila melanogaster has one pair of sex chromosomes (XX or XY) and three pairs of autosomes (II, III, IV). Knowing the chromosomal location of genes (e.g., black on chromosome II, pink on III) allows geneticists to deduce linkage groups by comparing expected and observed phenotypic ratios, especially when incorporating a new gene like short.